1.HashMap的底层机制
底层存放数据示意图：
k，v是一个Node实现了Map.Entry<K,V>
jdk8以上底层为数组+链表+红黑树
2.HashMap源码解读
解读代码：
Map map = new HashMap(); // 添加键值对 map.put("no1","dahe"); map.put("no2","zhangsan"); // Key重复会进行替换 map.put("no1","lisi");
新建HashMap对象，我们步入看看
首先，会进行初始化加载因子，加载因子的值为0.75
public HashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted } --- static final float DEFAULT_LOAD_FACTOR = 0.75f;
开始执行put方法，通过一通计算hash值，传入参数进入putVal方法
public V put(K key, V value) { return putVal(hash(key), key, value, false, true); }
putVal方法：（不要慌，详细的代码解读可以参见HashSet篇😁，接下来我们来debug看一下具体的代码执行过程）
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i; if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); else { Node<K,V> e; K k; if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; else if (p instanceof TreeNode) e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st treeifyBin(tab, hash); break; } if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } if (e != null) { // existing mapping for key V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) e.value = value; afterNodeAccess(e); return oldValue; } } ++modCount; if (++size > threshold) resize(); afterNodeInsertion(evict); return null; }
此时的table表为空。我们进行resize操作
if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length;
老样子，按规则进行扩容（规则详见HashSet篇）
final Node<K,V>[] resize() { Node<K,V>[] oldTab = table; int oldCap = (oldTab == null) ? 0 : oldTab.length; int oldThr = threshold; int newCap, newThr = 0; if (oldCap > 0) { if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; else { // zero initial threshold signifies using defaults newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } if (newThr == 0) { float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE); } threshold = newThr; @SuppressWarnings({"rawtypes","unchecked"}) Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; table = newTab; if (oldTab != null) { for (int j = 0; j < oldCap; ++j) { Node<K,V> e; if ((e = oldTab[j]) != null) { oldTab[j] = null; if (e.next == null) newTab[e.hash & (newCap - 1)] = e; else if (e instanceof TreeNode) ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); else { // preserve order Node<K,V> loHead = null, loTail = null; Node<K,V> hiHead = null, hiTail = null; Node<K,V> next; do { next = e.next; if ((e.hash & oldCap) == 0) { if (loTail == null) loHead = e; else loTail.next = e; loTail = e; } else { if (hiTail == null) hiHead = e; else hiTail.next = e; hiTail = e; } } while ((e = next) != null); if (loTail != null) { loTail.next = null; newTab[j] = loHead; } if (hiTail != null) { hiTail.next = null; newTab[j + oldCap] = hiHead; } } } } } return newTab; }
接下来，通过hash值计算table的某个位置有没有数据存在，如果没有的话，新建一个Node节点，挂在table数组上
if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null);
这样一来，第一个数据就添加成功啦！
继续步入，直到第二个元素添加完成，关键来了，开始添加第三个元素
步入步入，到达比较hash值的部分，由于新增的对象的hash值和第一个一致且key值相等，则执行e=p操作
if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p;
继续向下执行，会替换第一个键值对key所对应的值为新增的lisi
if (e != null) { // existing mapping for key V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) e.value = value; afterNodeAccess(e); return oldValue; }
